remove System.Collections.Immutable dependency (#666)

Author: SimonCropp

src/DiffEngine.Tests/GlobalUsings.cs

@@ -1,3 +1 @@
-global using System.Collections.Immutable;
-global using System.Management;
-global using EmptyFiles;
+global using EmptyFiles;

src/DiffEngine/DiffEngine.csproj

@@ -18,14 +18,11 @@
     <PackageReference Include="Caseless.Fody" PrivateAssets="All" />
     <PackageReference Include="EmptyFiles" PrivateAssets="None" />
     <PackageReference Include="Fody" PrivateAssets="all" />
-    <PackageReference Include="System.Collections.Immutable"
-                      Condition="'$(TargetFramework)' != 'net8.0' AND '$(TargetFramework)' != 'net9.0' AND '$(TargetFramework)' != 'net10.0'" />
     <PackageReference Include="Polyfill" PrivateAssets="all" />
     <PackageReference Include="System.Management" />
     <PackageReference Include="ProjectDefaults" PrivateAssets="all" />
   </ItemGroup>
   <Target Name="CheckPackageVersion" BeforeTargets="Build">
-    <Error Condition="'%(PackageVersion.Identity)' == 'System.Collections.Immutable' AND '%(PackageVersion.Version)' != '8.0.0'" Text="Invalid package version for System.Collections.Immutable. Expected: 8.0.0." />
     <Error Condition="'%(PackageVersion.Identity)' == 'System.Management' AND '%(PackageVersion.Version)' != '8.0.0'" Text="Invalid package version for System.Management. Expected: 8.0.0." />
   </Target>
 </Project>

src/Directory.Packages.props

@@ -14,7 +14,6 @@
     <PackageVersion Include="Polyfill" Version="9.3.2" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.163" />
     <PackageVersion Include="Resourcer.Fody" Version="1.8.1" />
-    <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" Pinned="true" />
     <PackageVersion Include="Serilog" Version="4.3.0" />
     <PackageVersion Include="Serilog.Sinks.File" Version="7.0.0" />
     <PackageVersion Include="System.Management" Version="8.0.0" Pinned="true" />